In a recording/reproduction apparatus for optical disks, in order to read data from an optical disk or write data to an optical disk, it is necessary to access a desired position on the optical disk by using an optical pickup. An optical pickup includes a light source for emitting a light beam, an objective lens for converging the light beam onto an optical disk, and a photodetector for outputting electrical signals based on the light beam having been reflected from the optical disk.
A semiconductor laser is used as a light source in an optical pickup. A semiconductor laser operates with a driving current which is supplied form a laser driving circuit, and is capable of emitting laser light of an intensity which is in accordance with the driving current.
Data which is recorded on an optical disk is reproduced by irradiating the rotating optical disk with a light beam having a relatively weak constant light amount, and detecting reflected light which has been modulated by the optical disk.
On a read-only optical disk, information in the form of pits is recorded in a spiral manner, previously during manufacture of the optical disk. On the other hand, in the case of a rewritable optical disk, a method such as vapor deposition is used to deposit a film of recording material which allows for optical data recording/reproduction, on the surface of a base material on which a track having spiral land or groove is formed. In the case where data is to be recorded on a rewritable optical disk, the optical disk is irradiated with a light beam whose light amount is modulated in accordance with the data to be recorded, thus causing local changes in the characteristics of the recording material film, whereby a data write is effected.
Note that the depth of the pits, the depth of the track, and the thickness of the recording material film are small relative to the thickness of the base material of the optical disk. Therefore, any portion of the optical disk where data is recorded constitutes a two-dimensional surface, and may be referred to as a “signal surface” or an “information surface”. Although such a signal surface (information surface) may sometimes need to be referred to as an “information layer” because of having a physical size along the depth direction, for simplicity, it will be referred to as a “signal surface (information surface)” in the present specification. An optical disk includes at least one such signal surface. Note that one signal surface may actually include a plurality of layers, e.g., a phase-change material layer and a reflective layer.
When reproducing data which is recorded on an optical disk, or recording data onto a recordable optical disk, it is necessary for a light beam to always retain a predetermined convergence state on a target track on the signal surface. This requires “focus control” and “tracking control”. “Focus control” refers to controlling the position of an objective lens along a normal direction of the signal surface (hereinafter may be referred to as the “depth direction of the substrate”) so that a focal point of the light beam (convergence point) is always positioned on the information layer. On the other hand, tracking control refers to controlling the position of an objective lens along a radial direction of the optical disk (hereinafter referred to as the “disk radial direction”) so that a spot of the light beam is positioned on a predetermined track.
In order to perform focus control and tracking control, it is necessary to generate a focus error signal and a tracking error signal based on light which is reflected from the signal surface of an optical disk. Such signals are generated based on electrical signals which are output from a photodetector which is provided within an optical pickup.
Out of the light which is reflected from the optical disk (hereinafter referred to as “reflected light”), unnecessary light, which is other than the portion which is necessary for generating various signals, may enter the photodetector. If scratches or dust is present on the surface of the optical disk, light may be scattered or reflected in unexpected directions when the convergence point of the light beam which is emitted from the optical pickup crosses the scratches or dust. When the unnecessary light which is thus generated enters the photodetector as stray light which does not follow a predetermined path, the output signal from the photodetector may have a noise component.
Patent Document 1 and Patent Document 2 disclose optical pickups which have an optical shield for blocking such stray light.
FIG. 7 is an essential structural diagram showing a conventional optical pickup which is described in Patent Document 1.
The illustrated optical pickup includes an integrated light receiving/emitting element 101, an objective lens 103 for converging the light which is emitted from the integrated light receiving/emitting element 101 onto a signal surface of an optical disk 104, and an optics base 107 on which the integrated light receiving/emitting element 101 and the like are fixed.
The integrated light receiving/emitting element 101 is an element in which the followings are integrated: a light source 101d for emitting light with which the optical disk 104 is irradiated; a photodetector 101c for detecting light reflected from the signal surface of the optical disk 104 and converting it to electrical signals; and a hologram element 101a having a hologram region 101b for guiding the reflected light into the photodetector 101c. 
This optical pickup includes an light-shielding mask 108 which has an aperture 108a in the center.
Light which is emitted from the light source 101d travels through the aperture 108a of the light-shielding mask 108, and thereafter is converged by the objective lens 103 onto a signal surface of the storage medium 104. The light which is reflected from the signal surface of the optical disk 104 again travels through the objective lens 103 and the aperture 108a of the light-shielding mask 108, and is diffracted by the hologram region 101b of the hologram element 101a, thus being converged onto the photodetector 101c. The photodetector 101c outputs a reproduction signal from the optical disk 104, as well as a focus error signal and a tracking error signal with which to control the position of the objective lens 103.
The light-shielding mask 108 functions to block any light other than a light beam 105 which is necessary for generating the various signals mentioned above so that the light does not enter the photodetector 101c. The reason is that, if reflected light from the optics base 107 and the objective lens 103 and unnecessary light other than the reproduced information from the optical disk 104 returned to the photodetector 101c, the noise component in the various signals would increase. In order to reduce the unnecessary light causing such noise as much as possible, the aperture 108a of the light-shielding mask 108 is designed so as to be as small as possible.
FIG. 8 is an essential structural diagram showing a conventional optical pickup which is described in Patent Document 2. The optical pickup of FIG. 8 includes an integrated light receiving/emitting element 201, and an objective lens 203 for converging light which is emitted from the integrated light receiving/emitting element 201 onto the signal surface of the optical disk 204.
The integrated light receiving/emitting element 201 is an element in which the followings are integrated: a light source 201d for emitting light with which the optical disk 204 is irradiated; a photodetector 201c for detecting light reflected from the signal surface of the optical disk 204 and converting it to electrical signals; and a hologram element 201a for guiding the reflected light into the photodetector 201c. 
This optical pickup includes an light-shielding mask 209 which has an aperture 209a in the center.
The light-shielding mask 209 covers five faces of the hologram element 201a, and is capable of blocking almost all of the unnecessary light that may enter the photodetector 201c. The light-shielding mask 209 is directly attached to the hologram element 201a. Therefore, the positioning precision of the aperture 209a with respect to the hologram region of the hologram region 201 is high, and the aperture 209a can be made small without blocking any light which is necessary for irradiating the storage medium 204.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 11-25496
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2001-110085